High Efficiency Video Coding (HEVC) is a new video coding standard currently being developed in Joint Collaborative Team-Video Coding (JCT-VC). JCT-VC is a collaborative project between Moving Picture Experts Group (MPEG) and International Telecommunication Union—Telecommunication Strandardization Sector (ITU-T). Currently, an HEVC Model (HM) is defined that includes a number of new tools and is considerably more efficient than H.264/Advanced Video Coding (AVC).
A picture in HEVC is partitioned into one or more slices, where each slice is an independently decodable segment of the picture. This means that if a slice is missing, for instance got lost during transmission, the other slices of that picture can still be decoded correctly. In order to make slices independent, they do not depend on each other. No bitstream element of another slice is required for decoding any element of a given slice.
Each slice contains a slice header which independently provides all required data for the slice to be independently decodable. One example of a data element present in the slice header is the slice address, which is used for the decoder to know the spatial location of the slice. Another example is the slice quantization delta which is used by the decoder to know what quantization parameter to use for the start of the slice. There are many more data elements in the slice header.
HEVC also has mechanisms for handling reference pictures, which are previously decoded pictures to be used for decoding a current picture. The reference pictures are included in reference picture lists, which for HEVC is similar to the reference picture list in H.264.
In the draft HEVC specification three types of slices are defined:                I slices, also referred to as slice type 2, which does not use inter prediction and does not have any reference picture lists;        P slices, also referred to as slice type 0, which has only one reference picture list L0; and        B slices, also referred to as slice type 1, which has three reference picture lists L0, L1 and LC.        
In H.264, the reference picture lists L0 and L1 are first initialized. Then there is an optional modification process that is controlled by bitstream syntax elements that are present in the bitstream.
It has been proposed that HEVC should have three reference picture lists that are initialized similarly to H.264. They are called L0, L1 and LC. Also here there are syntax elements to modify the reference picture lists. The reference picture lists are then used in the decoding process of the current slice in the current picture.
For both H.264 and HEVC there is the possibility to modify the reference picture lists independently for each slice. That is done by indicating how the reference picture lists should be modified according to a specified syntax. One slice may for instance use the initialized reference picture lists without modification while for another slice there are syntax elements present that change the reference picture lists of the slice and make them different to the initialized reference picture lists.
The decoding process to generate final reference picture lists is as follows:                1. The decoder constructs initial reference picture lists. This is also referred to as the decoder initializes the reference picture lists.        2. The decoder checks whether optional list modification syntax elements are present in the bitstream. This is done by checking the values of list modification flags that are present in the bitstream.        3. If there are no list modification syntax elements present according to step 2, the initial reference picture lists from step 1 are used in the decoding process of the current slice. If there are list modification syntax elements according to step 2, the decoder decodes list modification syntax elements and performs list modification operations on the initial reference picture lists from step 1. These modified reference picture lists are then used in the decoding process of the current slice.        
A problem with the prior art technique is that the reference picture handling implies complex hardware decoder designs in order to be able to provide full flexibility of slices and handle the list initialization and modification processes. A hardware decoder will typically run the list construction on a slow processor and not with dedicated hardware. If an encoded picture is split into a high number of slices and modifications are used for the slices, the decoding complexity of list construction will be significant.